Method and apparatus for treating Cheyne-Stokes respiration

ABSTRACT

A system and method for delivering a flow of breathing gas to an airway of a patient. The system monitors a characteristic that varies based on variations of the flow of the breathing gas and determines a Target Flow for the gas to be delivered to the patient based on the monitored characteristic. The Target Flow is set to a level sufficient to treat Cheyne-Stokes respiration or a sleep disordered breathing event. The system also alters the Target Flow based on a determination that the patient is experiencing a sleep disordered breathing event. The system further determines a pressure relief component and controls the flow of gas delivered to the patient based on the Target Flow and the pressure relief component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 12/751,081, filed Mar. 31, 2010, now U.S. Pat. No.8,220,456, which is a Continuation under 35 U.S.C. §120 of U.S. patentapplication Ser. No. 11/235,520, filed Sep. 26, 2005, now U.S. Pat. No.7,717,110, which claims priority under 35 U.S.C. §119(e) fromprovisional U.S. patent application No. 60/615,328, filed Oct. 1, 2004,the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a method and apparatus forproviding a positive pressure therapy particularly suited to treatCheyne-Stokes respiration and other breathing disorders commonlyassociated with congestive heart failure.

2. Description of the Related Art

Congestive heart failure (CHF) patients commonly suffer from respiratorydisorders, such as obstructive sleep apnea (OSA) or central apneas.Another such respiratory disorder CHF patients often experience duringsleep is known as Cheyne-Stokes respiration. FIG. 1 illustrates atypical Cheyne-Stokes respiration (CSR) pattern 30, which ischaracterized by rhythmic waxing periods 32 and waning periods 34 ofrespiration, with regularly recurring periods of high respiratory drive(hyperpnea) 36 and low respiratory drive (hypopnea or apnea) 38. Atypical Cheyne-Stokes cycle, generally indicated at 40, lasts about oneminute and is characterized by a crescendo (arrow A), in which the peakrespiratory flow of the patient increases over several breath cycles,and decrescendo (arrow B), in which the peak respiratory flow of thepatient decreases over several breath cycles. The typical Cheyne-Stokescycle ends with a central apnea or hypopnea following the decrecendophase. Apneas, hyperpneas, and the abnormal change in the depth and rateof breathing often cause arousals and, thus, degrades sleep quality.This disruption in sleep, as well as the periodic desaturation ofarterial oxygen (PaO₂), caused by the CSR cycle stresses thecardio-vascular system and specifically the heart.

The earliest treatment for CSR involved stimulating the respiratorydrive by administering Theophyline, caffeine, or 1-3% inspired carbondioxide to the patient. Although sometimes effective in reducing CSR,the downside of these treatments, which increase the respiratory rate,is that the increase in respiratory rate proportionally increasescardiac and respiratory workload.

Recent work in the treatment of sleep apnea has included the use of acontinuous positive airway pressure (CPAP) therapy in which a relativelyconstant positive airway pressure is delivered to the airway of apatient. Positive airway pressure therapy has been applied not only tothe treatment of breathing disorders, such as OSA, but also has beenused in the treatment of CHF. The effect of the CPAP therapy when usedto treat CHF is to raise the pressure in the chest cavity surroundingthe heart and allows cardiac output to increase.

Bi-level positive airway therapy has also been advanced in the treatmentof sleep apnea and related breathing disorders. In bi-level therapy,pressure is applied alternately at relatively higher and lowerprescription pressure levels within the airway of the patient so thatthe therapeutic air pressure is alternately administered at a larger andsmaller magnitude. The higher and lower magnitude positive prescriptionpressure levels are known as inspiratory positive airway pressure (IPAP)and expiratory positive airway pressure (EPAP), respectively.

Some preliminary investigations reveal that cardiac output improves whenpatients are supported using bi-level pressure therapy. It has also beenrecognized that CSR can be treated by augmenting respiratory effort withpressure support when the CSR pattern is in hypopnea region 38. Toaccomplish this, it is known to use a ventilator or pressure supportsystem to deliver machine triggered breaths during the hypopnea intervalwhen the patient's own respiratory drive is reduced or not present. Itis also known to treat CSR by decreasing the ventilatory efficiency whenflow is in a hyperpnea region 36. For example, published PCT Appln. No.WO 00/45882 teaches using rebreathing during a hyperpnea region toreduce the patient's ventilatory effectiveness, much the same way aperson hyperventilating is coached to breathe into a paper bag.

Yet another approach to providing therapy for the treatment of CSR isdescribed in U.S. Pat. No. 6,532,959 (“the '959 patent”). According tothe teachings of this patent, patients are provided with ventilatorysupport using a blower and mask. The system taught by the '959 patentdetermines a parameter referred to as “instantaneous ventilation”, whichis derived by measuring the volume inspired and the volume expired overa short period of time, calculating the average of the two, and thendividing this result in half. This derived instantaneous ventilation isused to adjust the level of ventilatory support by comparing theinstantaneous to a target volume that is determined from a long-termaverage of the patient's respiratory volumes, i.e., an average of thevolumes of the last 1-2 minutes. In theory, the short-term instantaneousventilation will be less than the long-term target during a hypopneaphase of the CSR cycle. As a result, the ventilatory support to thepatient's respiration is increased. The opposite result will occurduring the hyperpnea phase of the CSR cycle.

One disadvantage of the method of treating CSR taught by the '959 patentis that in many cases, the average value of the past respiratory volumesdoes not produce a target volume that will result in sufficienttreatment of the hypopneas and apneas. CSR has a continuum of severityand, depending on the level of severity, the target volume will need tobe adjusted to values other than the average of the last 1-2 minutes.Moreover, the CHF patient may have some degree of airway obstructionthat must be treated for its own sake, but it also must be treatedbecause these obstructive events appear to drive the CSR pattern aswell. Therefore, a simple system that sets the target volume based on along-term average of the past volumes does not address the interplay ofobstructing airways and CSR. It should also be noted that periodic legmovements, prevalent in 60%-80% of CHF patients, are also suspected todrive the CSR pattern. The volume calculation used by the '959 patent isalso prone to errors due to small bias errors in the estimated patientflow and to detecting the onset and termination of inspiration.

Another CSR treatment technique is disclosed in U.S. Pat. No. 6,752,151(“the '151 patent”). This patent describes a CSR detection and treatmenttechnique that monitors the peak flow in a pressure support systemcoupled to a patient to determine whether that patient is experiencingCSR. If so, the '151 patent teaches increasing IPAP, EPAP, or both totreat the CSR pattern. Detecting CSR based on the peak flow is believedto be more reliable than detecting CSR based on measured volumes,because the effect of an error in the estimated patient flow is alwayssmaller in a peak flow determination than that in a volume calculation.

One embodiment of the variable positive airway pressure technique taughtby the '151 patent teaches changing a pressure support level based on acomparison between a current peak flow and a target peak flow. Thepressure support level (PS) is the difference between the IPAP and EPAPlevels. The algorithm for changing the pressure support for a new breath(PS(k+1)) is given in the '151 patent as follows:PS(k+1)=PS(k)+Gain*(Target Flow−Q _(pk)(k)),  (1)where: k is the index of the pervious breath, PS(k) is the pressuresupport level for the previous breath, Gain is a factor that convertsflow into pressure, Target Flow is the target peak flow, and Q_(pk)(k)is the peak flow from the previous breath.

The '151 patent teaches adjusting the pressure support on abreath-by-breath basis such that the peak flow is at least as high asthe target peak flow. The result is that pressure support increases whenthe flow is in the hypopnea region and decreases to zero while flow isin the hyperpnea region. The pressure support is synchronized to patienteffort when present. During a central apnea, the '151 patent teachesdelivering machine triggered breaths at a predetermined rate andduration.

The '151 patent further teaches adjusting the target flow based on theeffectiveness of the pressure support therapy and determines the degreeof pressure support intervention. More specifically, the following threeparameters are monitored: 1) a CSR shape index, 2) a CSR severity index,and 3) a pressure support (PS) index. Based on these criteria, thetarget peak flow and/or the EPAP level are adjusted.

While the '151 patent teaches a robust and reliable technique fortreating CSR, the present inventors recognized that there may be someshortcomings with this technique. For example, the '151 patent monitorsthe actual flow Q_(pk)(k) in determining the pressure support and inanalyzing the effectiveness of the CSR treatment. However, this actualpeak flow may include anomalies that can introduce errors in thecalculations performed by the device taught by this patent. In addition,the technique taught by the '151 patent for selecting the Target Flowmay not maximize effectiveness in controlling the pressure support.Furthermore, the '151 patent does not adjust the pressure during abreath to ensure that the patient receives the necessary pressure orflow during each breath or to prevent the patient from receiving toohigh a pressure or flow during that respiratory cycle.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide apressure support system adapted to treat CSR that overcomes theshortcomings of conventional CSR treatment techniques. This object isachieved according to one embodiment of the present invention byproviding a pressure support system that includes a gas flow/pressuregenerating system that generates a flow of breathing gas, and a patientcircuit coupled to the gas flow/pressure generating system and adaptedto communicate the flow of breathing gas to an airway of a patient. Amonitoring means is provided to monitor a characteristic that variesbased on variations of the flow of the breathing gas, such as flow. Acontroller determines a Target Flow to be delivered to the patient basedon the monitored characteristic. The Target Flow is set to a levelsufficient to treat Cheyne-Stokes respiration or a sleep disorderedbreathing event. The controller determines whether such a patient isexperiencing a sleep disordered breathing event and alters the TargetFlow based on this determination. The controller also determines apressure relief component. The controller controls the gas flow/pressuregenerating system based on the Target Flow and the pressure reliefcomponent.

It is yet another object of the present invention to provide a method ofdelivering pressurized breathing gas to an airway of a patient that doesnot suffer from the disadvantages associated with conventional pressuresupport techniques. This object is achieved by providing a method thatincludes (a) delivering a flow of gas to the airway of the patient froma source of breathing gas via a patient circuit, (b) monitoring acharacteristic that varies based on variations of the flow of thebreathing gas. (c) determining a Target Flow to be delivered to thepatient based on the monitored characteristic, wherein the Target Flowis set to a level sufficient to treat Cheyne-Stokes respiration or asleep disordered breathing event, (d) determining whether such a patientis experiencing a sleep disordered breathing event, (e) altering theTarget Flow based on a determination that such a patient is experiencinga sleep disordered breathing event, (f) determining a pressure reliefcomponent, and (g) controlling the flow of breathing gas based on theTarget Flow and the pressure relief component.

It is a further object of the present invention to provide a system andmethod for delivering a machine triggered breath in an optimal fashionthat can be used alone or in combination with the above-describedinventions. This technique includes monitoring a first amount of timethat has elapsed between (a) a transition from an expiratory phase to aninspiratory phase of a respiratory cycle and (b) a transition from theinspiratory phase to an expiratory phase of the respiratory cycle(T_(insp)). An apnea detection time T_(apnea) is determined asT_(insp)+a constant. The system monitors a second amount of time thathas elapsed since the transition from the expiratory phase to theinspiratory phase of a respiratory cycle, and compares the second amountof time to T_(apnea). The machine triggered breath is provided when thesecond amount of time reaches T_(apnea).

Another object of the present invention is to provide a system andmethod that allows the pressure delivered to the patient to be alteredduring the inspiratory phase of a respiratory cycle. For example, thepresent invention contemplates monitoring a characteristic that varieswith changes in flow, and increasing the pressure of the flow ofbreathing gas if a Target Flow will not be met, or decreasing thepressure if the Target Flow will be exceeded, based on the monitoredcharacteristic.

These and other objects, features, and characteristics of the presentinvention, as well as the methods of operation and functions of therelated elements of structure and the combination of parts and economiesof manufacture, will become more apparent upon consideration of thefollowing description and the appended claims with reference to theaccompanying drawings, all of which form a part of this specification,wherein like reference numerals designate corresponding parts in thevarious figures. It is to be expressly understood, however, that thedrawings are for the purpose of illustration and description only andare not intended as a definition of the limits of the invention. As usedin the specification and in the claims, the singular form of “a”, “an”,and “the” include plural referents unless the context clearly dictatesotherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical Cheyne-Stokes respiratory cycle that istreated by the pressure support system of the present invention;

FIG. 2 is a functional block diagram of a positive airway pressuresupport system adapted to implement the pressure support therapyaccording to the principles of the present invention;

FIGS. 3A and 3B illustrate exemplary pressure waveforms delivered by thepressure support system of FIG. 2 according to the principles of thepresent invention;

FIG. 4 is a flowchart illustrating a portion of the process forimplementing the pressure support mode of the present invention;

FIG. 5 is a flow waveform illustrating the calculation of InstantaneousAverage Inspiratory Flow and Maximum Average Inspiratory Flow from theestimated patient flow;

FIG. 6A is a chart showing an array of peak flow data collected by thevariable positive airway pressure support system, FIG. 6B illustratesthe array of peak flow data after a first DC bias removal process, andFIG. 6C illustrates the array of peak flow data normalized forcomparison to an exemplary CSR template waveform used by the system togauge the effectiveness of the pressure support treatment;

FIG. 7 is an example of a normal distribution curve for an array ofMaximum Average Inspiratory Flows;

FIG. 8 is a state diagram explaining the Target Flow selection processaccording to the principles of the present invention;

FIGS. 9A-9D are waveforms illustrating the operation of the pressuresupport system of the present invention;

FIG. 10 is a flowchart illustrating the process carried out during eachrespiratory cycle according to the principles of the present invention;

FIG. 11 is a flowchart illustrating the intra-breath IPAP pressurecontrol technique according to the principles of the present invention;

FIG. 12A is an exemplary pressure waveform and FIG. 12B is acorresponding exemplary flow waveform showing the intra-breath pressurecontrol technique implemented according to the process of FIG. 11;

FIG. 13 is an exemplary flow waveform showing the intra-breath pressureincrease technique according to the principles of the present invention;

FIG. 14 is a flowchart illustrating the intra-breath pressure controltechnique according to the principles of the present invention;

FIGS. 15A-15C are waveforms illustrating the intra-breath pressurecontrol technique according to the principles of the present invention;

FIG. 16 is a flowchart illustrating the machine triggered breathpressure delivery technique according to the principles of the presentinvention;

FIGS. 17A-17C illustrate various alternative situations for deliveringmachine triggered breaths according to the process of FIG. 16; and

FIG. 18 is a state diagram illustrating the oxygen saturation states ofa patient experiencing a CSR cycle.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS OF THEINVENTION

I. System Hardware

FIG. 2 schematically illustrates a positive airway pressure supportsystem 50 suitable for providing an improved variable positive airwaypressure mode of pressure support to a patient according to theprinciples of the present invention. This mode of pressure support isparticularly suited to treat Cheyne-Stokes respiration. Pressure supportsystem 50 includes a gas flow/pressure generator 52, such as a blowerused in a conventional CPAP or bi-level pressure support device, piston,bellows, compressor, or any other device that receives breathing gas,generally indicated by arrow C, from any suitable source, e.g., apressurized tank of oxygen or air, the ambient atmosphere, or acombination thereof. Gas flow/pressure generator 52 generates a flow ofbreathing gas, such as air, oxygen, or a mixture thereof, for deliveryto an airway of a patient 54 at relatively higher and lower pressures,i.e., generally equal to or above ambient atmospheric pressure.

The pressurized flow of breathing gas, generally indicated by arrow Dfrom gas flow/pressure generator 52 is delivered, via a delivery conduit56, to a breathing mask or patient interface 58 of any knownconstruction, which is typically worn by or otherwise attached to apatient 54 to communicate the flow of breathing gas to the airway of thepatient. Delivery conduit 56 and patient interface device 58 aretypically collectively referred to as a patient circuit.

Although not shown in FIG. 2, the present invention also contemplatesproviding a secondary flow of gas, either alone or in combination withthe primary flow of gas (arrow C) from atmosphere. For example, a flowof oxygen from any suitable source, such as an oxygen concentrator, oroxygen storage device (liquid or gas), can be provided upstream of gasflow/pressure generator 52 or downstream of the gas flow generator, forexample, in the patient circuit or at the patient interface device, tocontrol the fraction of inspired oxygen delivered to the patient.

Pressure support system 50 shown in FIG. 2 is a single-limb system,meaning that the patient circuit includes only a delivery conduit 56connecting the patient to the pressure support device. As such, anexhaust vent 57 is provided in the delivery conduit for venting exhaledgasses from the system as indicated by arrow E. It should be noted thatthe exhaust vent can be provided at other locations in addition to orinstead of in the delivery conduit, such as in the patient interfacedevice. It should also be understood that the exhaust vent can have awide variety of configurations depending on the desired manner in whichgas is to be vented from the pressure support system.

The present invention also contemplates that the variable positiveairway pressure support system can be a two-limb system, having adelivery conduit and an exhaust conduit connected to the patient. In atwo-limb system, the exhaust conduit carries exhaust gas from thepatient and includes an exhaust valve at the end distal from thepatient. The exhaust valve is typically actively controlled to maintaina desired level of pressure in the system, which is commonly known aspositive end expiratory pressure (PEEP). This is accomplished bycontrolling the flow of exhaust gas from the otherwise closed system.

In the illustrated exemplary embodiment of the present invention,patient interface 58 is a nasal/oral mask. It is to be understood,however, that patient interface 58 can include a nasal mask, nasalpillows, tracheal tube, endotracheal tube, or any other device thatprovides the gas flow communicating function. Also, for purposes of thepresent invention, the phrase “patient interface” can include deliveryconduit 56 and any other structures that connect the source ofpressurized breathing gas to the patient.

It is to be understood that various components may be provided in orcoupled to the patient circuit. For example, a bacteria filter, pressurecontrol valve, flow control valve, sensor, meter, pressure filter,humidifier and/or heater can be provided in or attached to the patientcircuit. Likewise, other components, such as muffler and filters can beprovided at the inlet of gas flow/pressure generator 52 and at theoutlet of valve 60.

In the illustrated embodiment, variable positive airway pressure supportsystem 50 includes a pressure controller in the form of a valve 60provided in delivery conduit 56. Valve 60 controls the pressure of theflow of breathing gas from gas flow/pressure generator 52 delivered tothe patient. For present purposes, gas flow/pressure generator 52 andvalve 60 are collectively referred to as a “pressure generating system”because they act in concert to control the pressure and/or flow of gasdelivered to the patient.

It should be apparent that other techniques for controlling the pressuredelivered to the patient by the gas flow/pressure generator, such asvarying the blower speed, either alone or in combination with a pressurecontrol valve, are contemplated by the present invention. Thus, valve 60is optional depending on the technique used to control the pressure ofthe flow of breathing gas delivered to the patient. If valve 60 iseliminated, the pressure generating system corresponds to gasflow/pressure generator 52 alone, and the pressure of gas in the patientcircuit is controlled, for example, by controlling the motor speed ofthe gas flow/pressure generator.

Pressure support system 50 further includes a flow sensor 62 thatmeasures the flow of breathing gas within delivery conduit 56. Inaccordance with a presently preferred embodiment shown in FIG. 2, flowsensor 62 is interposed in line with delivery conduit 56, mostpreferably downstream of valve 60. Flow sensor 62 generates a flowsignal Q_(measured) that is provided to a controller 64 and is used bythe controller to determine the flow of gas at the patient Q_(patient).

Techniques for calculating Q_(patient) based on Q_(measured) are wellknown, and take into consideration the pressure drop of the patientcircuit, known leaks from the system, i.e., the intentional exhaustingof gas from the circuit as indicated by arrow E in FIG. 2, and unknownleaks from the system, such a leaks at the mask/patient interface. Thepresent invention contemplates using any conventional technique forcalculating leak flow Q_(leak), and using this determination incalculating Q_(patient) based on Q_(measured). Examples of suchtechniques are taught by U.S. Pat. Nos. 5,148,802; 5,313,937; 5,433,193;5,632,269; 5,803,065; 6,029,664; 6,539,940; and 6,626,175, and by U.S.patent application Ser. No. 10/243,016, publication no. US-2003-0066528,the contents of each of which are incorporated by reference into thepresent invention.

Other techniques for measuring the patient flow of the patient arecontemplated by the present invention. For example, the flow can bemeasured directly at the patient, in which case the measured flowcorresponds directly the patient flow Q_(patient) and no flow estimationis necessary. The present invention also contemplates measuring the flowat other locations along delivery conduit 56.

In addition, the present invention contemplates determining theestimated patient flow Q_(patient) based on other characteristics of thepressure support system. For example, the operation of the gasflow/pressure generator or a flow/pressure controller, such as a valve,is affected by the flow in the patient circuit, or by the systemsattempt to maintain the pressure in the system. As a result, monitoringa characteristic of the system, such as monitoring the power, torque,and/or rotating speed of the pressure generator or the position of thevalve, can be used as a surrogate for measuring the patient flowdirectly. It is also known to measure patient flow using a flow sensorupstream of the gas flow/pressure generator. Of course, any combinationof such flow measuring techniques can also be used. In these lattercases, an estimation of patient flow Q_(patient) based on the measuredflow or other parameter will be needed.

An input/output device 66 is provided for setting various parametersused by the variable positive airway pressure support system, as well asfor displaying and outputting information and data to a user, such as aclinician or caregiver. It is to be understood that the presentinvention contemplates providing input/output terminals so that theoperation information and data collected by the pressure support systemcan be monitored and controlled remotely. Controller 64 is preferably amicroprocessor that is capable of implementing and executing routinesfor monitoring characteristics of patient respiration and controllingthe flow of breathing gas based thereon as discussed in detail below. Inaddition, controller 64 includes memory, or memory arrays for storingand buffering information necessary to implement the techniquesdiscussed herein. It is to be understood, that controller 64 can be asingle processing component, or can be comprised of multiple components(memories, processor, arrays, logic circuits, etc.) operating inconjunction to implement the techniques discussed herein.

II. Pressure Support to the Patient

In a preferred embodiment of the present invention, controller 64controls gas flow/pressure generator 52, valve 60, or both to deliver apressure waveform to an airway of patient 54. In an exemplary embodimentof the present invention, the pressure waveform is essentially abi-level pressure waveform that alternates between an IPAP level and anEPAP level. See FIG. 3A. According to the present invention, the IPAPlevel is variable under the direction of controller 64 as discussedbelow. The maximum and minimum IPAP levels (IPAP_(max), IPAP_(min)) areprovided to the controller via input device 66 from a user. It should beunderstood that the maximum and minimum IPAP levels can also bepre-established and stored in the controller as a default or in lieu ofinput parameters from the system operator. The present invention alsocontemplates setting the EPAP level manually or pre-established.

FIGS. 3A and 3B illustrate exemplary pressure waveforms 76 and 78 thatcan be provided by the pressure support system to treat CSR. As shown inFIGS. 3A and 3B, at time F, which is the trigger point from expirationto inspiration, the patient begins inspiring and triggers the pressuresupport system to transition to an IPAP level 80. The shape and durationof the pressure increase or rise 82 from trigger point F to the IPAPlevel can be fixed or variable, as taught for example, in U.S. Pat. Nos.5,598,838; 5,927,274; 6,532,960; and 6,640,806, the contents of each ofwhich are incorporated herein by reference. In the illustratedembodiment, the shape of the pressure increase is exponential. It is tobe understood that other shapes, such as step functions or linear rampsare contemplated for the pressure rise portion of an inspiratory portion83 of the pressure waveform.

It should be further understood that the present invention contemplatesthat an inspiratory portion 83 of pressure waveform 76 can have avariety of configurations. That is, the pressure waveform duringinspiration P_(insp) can be controlled using conventional pressuresupport or ventilation techniques, such as proportional assistventilation (PAV®), which is described in U.S. Pat. Nos. 5,044,362 and5,107,830, or proportional positive airway pressure (PPAP), which isdescribed in U.S. Pat. Nos. 5,535,738; 5,794,615; 6,105,575; and6,609,517 (“the PPAP patents”) the contents of each of which areincorporated herein by reference. According to the PPAP patents, thewaveform for inspiratory pressure, P_(insp), output by the pressuresupport system during the inspiratory phase of the breathing cycle isdetermined according to the following equation:P _(insp) =IPAP+Gain_(insp) *Q _(patient),  (2)where Gain_(insp) is a gain factor, typically selected by a caregiver.Gain_(insp) can be set to any value including a value of one (1).

At time G in the pressure waveforms of FIGS. 3A and 3B, at the end ofthe inspiratory period, which is the cycle point from inspiration toexpiration, the patient begins the expiratory phase of the breathingcycle. At this point, the pressure support system cycles, causing thepressure to drop toward an EPAP level, indicated at 84. In theembodiment illustrated in FIG. 3A, the expiratory portion P_(exh) ofpressure waveform 76 corresponds to the expiratory pressure administeredby a conventional bi-level pressure support system, where the EPAP levelremains generally constant throughout the expiratory phase of thebreathing cycle once the pressure level hits the EPAP level.

It is to be understood that the present invention contemplates that theexpiratory portion P_(exh) of the pressure waveform can have a varietyof configurations and can be controlled using conventional pressuresupport or ventilation techniques, such as the PAV and PPAP techniquesnoted above. For example, FIG. 3B illustrates an exemplary embodimentfor the expiratory pressure, P_(exh), output by the pressure supportsystem in which the expiratory phase of the breathing cycle isdetermined according to the following equation:P _(exh) =EPAP+Gain_(exh) *Q _(patient),  (3)where Gain_(exh) is a gain factor, typically selected by a caregiver.Gain_(exh) can be set to any value including a value of one (1). ThePPAP patents teach this technique for controlling the expiratorypressure delivered by a bi-level pressure support system. As a result,the pressure delivered to the patient drops below EPAP at area H duringpatient exhalation, thereby increasing patient comfort. Controller 64receives flow Q_(measured) from flow sensor 62 and implements equations(2), (3), or both, for generating the inspiratory pressure waveformP_(insp) and expiratory pressure waveform P_(exh).III. Pressure Control Technique

Controller 64 implements an algorithm to control the pressure of theflow of gas delivered to the patient. Referring now to FIG. 4, a primaryinput to this algorithm is the output of flow sensor 62 (Q_(measured)).The output is sampled at a sampling rate, such as 100 samples/second, toproduce a new estimated patient flow Q_(patient) determination every 10milliseconds. As noted above, Q_(patient) is calculated based onQ_(measured) using known flow/leak estimation techniques. Of course,Q_(patient) can be measured directly at the mask so that flow estimationis not needed. The present invention also contemplates that the measuredflow Q_(measured) can be used directly for the calculations of thepresent invention, recognizing that the measured flow is not an accuraterepresentation of the flow at the airway of the patient.

A history of the patient flow Q_(patient) is stored in memory to performthe flow analysis discussed below. Controller 64 includes storage arraysand buffers to calculate parameters in real-time, and store the resultsin moving windows.

According to one aspect of the present invention, controller 64 monitorsthe patient flow to determine the transitions from inspiration toexpiration and from expiration to inspiration. While any suitabletechnique can be used for determining when trigger point F fromexpiration to inspiration and cycle point G from inspiration toexpiration, a presently preferred embodiment of the present inventionuses both volume and wave shape to (a) trigger the device to provide theinspiratory pressure P_(insp) and (b) cycle the device to provide theexpiratory pressure P_(exh). A volume trigger occurs when theaccumulated patient inspiratory volume exceeds a threshold level. Anexample of this is described in U.S. Pat. Nos. 5,148,802; 5,313,937; and5,433,193. Wave shape triggering refers to a triggering technique inwhich two waveforms, which are determined from a monitoredcharacteristic indicative of patient respiration, such as flow orpressure, are compared to one another. An example of this is describedin U.S. Pat. Nos. 5,632,269; 6,029,664; 6,539,940; and 6,626,175. Thoseskilled in the art will appreciate that cycling from inspiration toexpiration involves similar techniques.

It should be noted that for present purposes, flow into the patient isconsidered positive flow, and flow out of the patient is considerednegative flow. Thus, the value of the patient flow Q_(patient) is takenat the patient's airway. Those skilled in the art will appreciate thatthe flow measured at a location distal from the patient Q_(measured)will have a positive offset due, for example, to exhausting of gas fromthe circuit, which is factored out by the leak estimation techniques.

In step 100, in the flowchart shown in FIG. 4, the controller analyzesthe patient's instantaneous flow Q_(patient) to produce the followingtwo fundamental measures during a respiratory cycle. The first parameteris referred to as the Instantaneous Average Inspiratory Flow (Qave(t)).It is the summation of positive, i.e., inspiratory, patient flows over aperiod of time divided by the number of samples taken during that periodof time. An example of a patient flow Q_(patient) waveform 102 is shownin FIG. 5. A corresponding Instantaneous Average Inspiratory FlowQave(t) waveform 104 is also shown.

Instantaneous Average Inspiratory Flow is continuously calculated duringthe inspiratory phase of the respiratory cycle. The InstantaneousAverage Inspiratory Flow is used in the processes carried out by thepresent invention, rather than the patient flow Q_(patient) directly,because patient flow waveform 102 often is not clean. That is, thewaveform for Q_(patient) often contains spurious data and anomalies thatare equivalent to “noise” in an electrical signal. These anomalies aredue, for example, to noise in the sensor, movement of the patient, orphysiologic events or actions, such as snore, flow limitation, coughing,mucous build-up, changes in the patient's airways, or any combinationthereof. In effect, calculating the Instantaneous Average InspiratoryFlow Qave(t) acts to filter the patient flow to remove such “noise”.

The second parameter calculated during the inspiratory phase of therespiratory cycle is a Maximum Average Inspiratory Flow (Qave(max)),which is the maximum value 106 of the Instantaneous Average InspiratoryFlow over one breath, i.e., during the inspiratory phase of therespiratory cycle. It can thus be appreciated that during one giveninspiratory phase of a patient's respiratory cycle, a continuum ofQave(t) is calculated over the entire inspiratory phase, and only oneQave(max) is found. Again, the use of Qave(max), rather than an actualpeak, such as peak 108, of patient flow 102, is done because the patientflow Q_(patient) may include anomalies that, if not factored out, canresult in errors being carried throughout the calculations performed bythe present invention.

The level of Qave(max) during the inspiratory phase of each respiratorycycle is stored in a memory array in breath measures step 100 in FIG. 4.In addition to storing the Qave(max) for each breath, a time stampidentifying when the Qave(max) occurred, and an indication of the levelof pressure support being provided to the patient at that time are alsostored in the memory array. The pressure support (PS) is determined asthe difference between IPAP and EPAP. In other words, PS=IPAP−EPAP. Asdiscussed below, this stored information is used in other processes todetermine how well the pressure support system is functioning to treatCSR and sleep disordered breathing events and to adjust the systemparameters, if necessary.

A. Sleep Disordered Breathing Event Detection

In step 110, patient flow Q_(patient) is analyzed for evidence of sleepdisordered breathing events. The pressure control process is altered, asdiscussed below, depending on whether such events are present. Accordingto a presently preferred exemplary embodiment of the invention, thesystem monitors the patient flow Q_(patient) for the following events:CSR, hypopneas, apneas, and periodic breathing. The present inventionalso contemplates monitoring flow for other events indicative ofdisturbed breathing, such as snoring and flow limitation.

The present invention further contemplates that sleep disorderedbreathing events can be detected using inputs other than from the flowsensor or using other inputs in combination with the flow sensor. Forexample, snoring can be detected via a microphone. CSR, hypopneas,apneas, and periodic breathing can be detecting using other sensors,such as effort belts and thermister flow sensors.

1. CSR Detection

The following is a description of a presently preferred exemplaryembodiment for detecting CSR. As noted above, the present inventionmonitors for CSR to ensure that the pressure therapy being applied tothe patient is sufficient to treat CSR. Naturally, the presence of CSRindicates that the therapy is not effective. Thus, it is important thatCSR events be detected accurately and monitored. The steps discussedbelow are implemented in software run by the processor in the pressuresupport system. It is to be understood that the CSR detection techniquediscussed below represents one exemplary technique. The presentinvention contemplates and those skilled in the art would appreciatethat any suitable CSR detection technique can be used to monitor theeffectiveness in the CSR treatment delivered to the patient. See, e.g.,Section H below.

In the exemplary CSR detection technique of the present invention, thefollowing two fundamental measures are used to ascertain the presenceand severity of CSR in a patient: CSR Index and Flow Ratio. In general,historical patient flow data from the last 4 minutes is retained andanalyzed to determine these measures. The definitions for these measuresare as follows:

-   CSR Index—This is an indication as to how well the patient's flow    pattern matches a CSR template. This output is a number from 0    to 100. A value of 100 represents a perfect fit between the    patient's flow pattern to the CSR template. This value is expressed    in units of a percentage.-   Flow Ratio—This is a ratio of the Maximum Average Inspiratory Flow    (Qave(max)) for the smallest breath to the Qave(max) for the largest    breath during the monitored window of time. This output is a number    from 0 to 100. A value of 100 indicates that all breaths are the    same size. This value is expressed in units of a percentage.

The CSR index is determined based on a coherence function, which is amathematical tool for determining how well an unknown pattern is similarto a template pattern. In the present invention, the unknown pattern isa sequence of previously recorded Maximum Average Inspiratory FlowQave(max) values, and the template pattern is a pattern selected tocorrespond to a CSR pattern. The CSR index, expressed as a percentage,is a measure of how well these two patterns coincide, and, hence, howwell the Qave(max) data collected over the past several minutescorresponds to a CSR pattern; the closer the match, the more likely itis that the patient is experiencing CSR.

The coherence technique first requires acquiring the stored Qave(max)values over the last 4 minutes. The Qave(max) values are processed tofit a typical CSR pattern of at least one cycle, approximately 60 sec.in duration. Depending on the CSR template, this requires that theQave(max) values and time stamps for such values from the last 2-5minutes be stored in an array. Using a normalized cross-correlationtechnique, the Qave(max) values are compared to the CSR template, and aCSR index ranging from 0-100% is generated.

FIG. 6A illustrates an array of Qave(max) values 120 stored over thetime interval of interest, which is typically the last 2-5 minutes.Qave(max) values 120 are processed to remove the “DC” bias in this arrayof Qave(max) values, so that zero crossings 124 can be detected to yielda shifted array of Qave(max) values (Qave(max)′ 122 shown in FIG. 6B).

Removing the “DC” bias is accomplished, according to one embodiment ofthe present invention, by determining an average value of Qave(max) forthe array of Qave(max) values 120 and storing this average in an averagepeak flow array. In a presently preferred embodiment, the period of timefor this average is the last 4 minutes. The array of Qave(max) values isshifted downward by subtracting the average value of Qave(max) from eachQave(max) value in the array.

Of course, any conventional technique for effectively removing the DCbias, i.e., placing a zero line in the Qave(max) values 120 at theappropriate location, can be used, so long as it is then possible todetermine the zero crossings 124 of the shifted array of Qave(max)′values 122.

To find the zero crossings, the shifted array of Qave(max)′ values 122are searched, preferably starting at the most recent Qave(max)′ valueand working backwards in time, using a robust zero crossing detectionmethod with a 2 LPM hysteresis. The first three zero crossings 122having the same slopes are used to define the last two CSR cycles 126.Once a zero crossing is detected, it is also time-stamped.

From the zero crossing time-stamps, the period T_(CSR) of the CSR cycleis measured. The measured CSR periods are used to time-warp each of thetwo CSR cycles on to a CSR template 128. See FIG. 6C. Excessivetime-warp due to the measured CSR period being out of range, e.g., 40-90seconds, stops the process and a CSR Index of 0% (zero) is returned.Template 128 in FIG. 6C is a sequence of peak flows that describe thegeneral shape of CSR. In an exemplary embodiment of the presentinvention, a simple triangle function was used for this purpose. It isto be understood, however, that more complex or other functions can beused as the CSR template. To time-warp the shifted array of Qave(max)′values 122, the time stamps and the shifted array of Qave(max)′ valuesare used to map the shifted array of Qave(max)′ values on to the samesampling rate as the CSR template using linear interpolation. As aresult, a second array of Qave(max)″ values 130 is produced.

To perform the correlation in the discrete-time domain, i.e., usingdigital samples, the samples in the second array of Qave(max)″ values130 have to be time-aligned with those of the CSR template. Thecoherence function of Qave(max)″ values 130 to the CSR template 128 iscomputed. The result is called the CSR index which is given in percentand ranges from 0 to 100%.

In summary, the Qave(max) values are stored in an array along with thetimestamps of when the Qave(max) values occurred. Next, the first threezero-crossings are detected and the periods of the first two CSR cyclesare computed. The Qave(max) array is recalculated and time-warped inorder to fit the CSR template and the coherence function is computedyielding the CSR Index.

While the present invention describes the determination of the CSRindicated based on Qave(max), it is to be understood that the presentinvention also contemplates using a simple peak value (Q_(peak(max)))determined directly from the patient flow Q_(patient). This process isdescribed in U.S. Pat. No. 6,752,151, the contents of which areincorporated herein by reference.

The Flow Ratio is calculated from the array of Maximum AverageInspiratory Flows 120 (see FIG. 6A) as a ratio of the minimum Qave(max)over the maximum Qave(max) during the time interval of the array. Thelast minimum and maximum values for Qave(max), or an average of severalminimum and maximum values, occurring in the array of Qave(max) valuesduring the sample interval are used to determine the Flow Ratio, whichis expressed as a percentage. Stated mathematically, the Flow Ratio isgiven by:

$\begin{matrix}{{{{Flow}\mspace{14mu}{Ratio}} = {( \frac{{minimum}\mspace{14mu}{{Q{ave}}( \max )}}{{maximum}\mspace{14mu}{{Q{ave}}( \max )}} )*100}},} & (4)\end{matrix}$where the minimum Qave(max) value and the maximum Qave(max) value, arefound by the searching the Qave(max) values within the CSR periods. Theminimum Qave(max) values represent troughs in the CSR pattern(apnea/hypopnea periods 38 in FIG. 1), and the maximum Qave(max) valuesrepresent peaks in the CSR pattern (hyperpnea periods 36 in FIG. 1).Thus, the Flow Ratio provides an indication as to the severity of theCSR that the patient is suffering. In general, a Flow Ratio greater than50% is considered normal, less than 50% is abnormal, and an index of 0%indicates the occurrence of a central apnea. A Flow Ratio of 100indicates that all breaths during the time period of data stored in thearray are the same size.

The CSR Index and the Flow Ratio are used to determine whether thepatient is deemed to be experiencing CSR by comparing these values tothreshold levels. According to an exemplary embodiment of the presentinvention, if the CSR Index is greater than 75% and a Flow Ratio is lessthan 65%, the patient is deemed to be experiencing a CSR event. Thesethreshold values for the CSR Index and the Flow Ratio are empiricallydetermined based on observed data. It is to be understood that otherthreshold levels can be selected depending on the desired sensitivity tothe detection of CSR events.

It is to be further understood that the CSR index and the Flow Ratio canbe used individually to determine whether the patient is experiencing aCSR event. That is, the present invention contemplates determining onlythe CSR Index, for example, and comparing it to a threshold to determinewhether the patient is suffering from a CSR event.

Conversely, the present invention also contemplates taking intoconsideration other parameters in deciding whether or not a patient issuffering from a CSR event. For example, the present inventioncontemplates monitoring a Pressure Support Index, which is the percentof pressure support breaths during a monitored window that are 2 cmH₂Oover the minimum IPAP level. This output is a number from 0 to 100. Avalue of 100 indicates that all breaths inside the analysis window werepressure support breaths greater than 2 cmH₂O over the minimum IPAPlevel. This value is expressed in units of a percentage.

The Pressure Support Index (PS_(index)), unlike the CSR Index and theFlow Ratio, is not a measure of a parameter directly associated with theCSR cycle. Rather, the pressure support index is a measure of the amountof assistance that is being provided to the patient by the pressuresupport system in attempting to combat the CSR cycle, i.e., how much thepressure support system is intervening on behalf of the patient toaugment their ventilation.

The PS_(index) over a predetermined period of time is calculated asfollows:

$\begin{matrix}{{{PS}_{index} = {( \frac{\#{PS}_{thres}}{\#\mspace{14mu}{total}} )*100}},} & (5)\end{matrix}$where #PS_(thres) is the number of breaths where the pressure supportlevel was greater than or equal to a threshold value. In an exemplaryembodiment of the present invention this threshold value is IPAP_(min)+2cmH₂O. The # total is the total number of breaths over the period oftime of interest. In an exemplary embodiment, this period of time is thelast 4 minutes. This PS_(index) once determined, is preferably used tomeasure the level of ventilator assistance being provided to the patientby the pressure support/ventilatory system. The system can use thislevel of assistance in deciding what actions to take regarding changesin the patient's ventilation assistance.

2. Apnea and Hypopnea Detection

The present invention contemplates using any conventional technique fordetecting apneas and hypopneas. In its most basic form, apnea andhypopnea detection involves monitoring the patient flow Q_(patient) forreductions in flow below a threshold level for a predetermined period oftime. The threshold level and predetermined periods of time are levelsdeemed to constitute an apnea or hypopnea, i.e., meet the definition ofan apnea or hypopnea.

In a presently preferred embodiment of the present invention, the apneaand hypopnea detection techniques taught by published U.S. Patent Appln.No. US-2003-0111079-A1 (“the '079 application”) are used in step 110.The contents of the '079 application are incorporated herein byreference. However, in place of the weighted peak flow Q_(w) _(peak)used in the '079 application, the present invention uses peak to peakflow of the previous breath.

3. Periodic Breathing Detection

This measure examines the irregularity of the Maximum AverageInspiratory Flow Qave(max). If the patient is deemed to have too muchirregularity in the Qave(max), a periodic breathing event is declared.This event is considered a sleep disordered breathing event in step 110of FIG. 4. The present invention contemplates using any conventionaltechnique to determine when to declare a periodic breathing event. Anexample of such a technique is taught in the '079 application in thesection of this published application discussing the variable breathingcontrol layer. However, a brief description of this technique isprovided below for the sake of completeness.

Irregularity of the Maximum Average Inspiratory Flow is detected byperforming a statistical analysis on the scatter of the Qave(max) datacollected over a predetermined period of time to detect unstablebreathing patterns or abrupt changes in patient response. Morespecifically, in one embodiment of the present invention, the Qave(max)is monitored over a moving window, which in a presently preferredembodiment, is a four (4) minute window.

The standard deviation of the Qave(max) data collected during themonitoring window is calculated. The present invention contemplatesdetermining whether the Maximum Average Inspiratory Flow data issufficiently stable by calculating a periodic breathing number (VB#) asfollows:

$\begin{matrix}{{{{VB}\#} = {( \frac{{standard}\mspace{14mu}{deviation}}{mean} )*100}},} & (6)\end{matrix}$where the “mean” is the mean of the Qave(max) values over the monitoringwindow. The higher the VB#, the more variable the Qave(max) data.

In an exemplary embodiment of the present invention, if the VB# isgreater than 30%, the patient is deemed to be experiencing periodicbreathing. This threshold value is empirically determined based onobserved data. It is to be understood that other threshold levels can beselected depending on the desired sensitivity to the detection ofperiodic breathing events.

B. Statistical Measures

Referring again to FIG. 4, the algorithm uses statistical functions instep 140 to determine a level of ventilation which has been demonstratedby the patient over the last several minutes of breathing. The followingstatistical measures, based on the Maximum Average Inspiratory FlowQave(max), are calculated by controller 64 in step 140:

-   -   1) Mean,    -   2) 60^(th) percentile,    -   3) 95% of mean,    -   4) Standard Deviation, and    -   5) Standard Mean.

FIG. 7 illustrates an exemplary normal distribution of values forQave(max) around a mean 142 having a value of 30 with a standarddeviation of 4. In this example, 95% of the mean is 28.5 lpm and isindicated by line 144. The 60^(th) percentile of the data is 33.2 lpmand is indicated by line 146. Standard Mean is the ratio of StandardDeviation over the mean expressed as a percentage.

C. Target Flow Generation

Referring again to FIG. 4, the algorithm in step 150 determines a TargetFlow value that is used in determining the IPAP pressure to be deliveredto the patient by the pressure support system. As discussed in greaterdetail below, the Target Flow is a value against which a current MaximumAverage Inspiratory Flow Qave(max) is compared to determine whether theIPAP pressure should be changed. The Target Flow represents the value ofQave(max) that the pressure support system attempts to reach bycontrolling the IPAP pressure delivered to the patient. The presentinvention updates the Target Flow periodically, typically, on abreath-by-breath basis, to optimize the pressure support delivered tothe patient. The patient's Maximum Average Inspiratory Flow Qave(max) ismonitored against the Target Flow to determine whether the IPAPpressure, or some other characteristic associated with the inspiratorypressure P_(insp), should be altered to better treat the patient, and,in particular, the CSR cycles the patient may be experiencing.

According to a presently preferred exemplary embodiment, the Target Flowis selected from the statistical measures of the Maximum AverageInspiratory Flow (Qave(max)) discussed above with respect to step 140 inFIG. 4. That is, the Target Flow is taken as the 95% of the mean value,the 60^(th) percentile, or a value based on the Mean. The determinationof which of these statistical measures will be selected as the TargetFlow is determined based on the sleep disordered breathing eventsdetected in step 110.

According to one embodiment of the present invention, the Target Flow isselected to be the 95% of the mean value when the patient is stable, andthe 60^(th) percentile of the Qave(max) data points is used as theTarget Flow when sleep disordered breathing events (CSR, Hypopnea,Apnea, snoring, etc.) have occurred or the standard mean (PeriodicBreathing) is greater than 30%. The transition to a higher Target Flowvalue is done instantaneously when sleep disordered breathing events aredetected. On the other hand, the transition from a high to a low TargetFlow occurs two minutes after the events have subsided, and thetransition of the Target Flow is done over a set period of time, such as30 seconds. Of course, the present invention contemplates that thesetransitions can take place over other periods of time.

FIG. 8 is a state diagram showing, in detail, the process for selectingthe statistical measure to be used as the Target Flow. When the pressuresupport system is actuated, the Target Flow selection process starts atstep 200, where the Target Flow is set to a minimum value and initialdata is collected. In a current exemplary embodiment of the presentinvention, the minimum value for the Target Flow is determinedempirically. In the present embodiment, this minimum Target Flow is setto 15 lpm. It is to be understood, however, that the present inventioncontemplates that the Target Flow can be set by the system based onmonitored physiological characteristics of the patient, such as whetherthe patient is deemed to be experiencing sleep disordered breathing,flow limitations, etc.

In step 202, the Target Flow is increased to (1) a value thatcorresponds to 95% of the mean value of the Qave(max) data thuscollected or to (2) a value that corresponds to the mean value ofQave(max) minus a fixed flow rate, which ever is smaller. In anexemplary embodiment of the present invention, this fixed flow rate is 2lpm. In a presently preferred exemplary embodiment, the increase in theTarget Flow is done in a linear, ramp fashion over a period of time thatspans several respiratory cycles, such as 30 seconds. This ramp in theTarget Flow is done to avoid rapid pressure fluctuations beingintroduced to the patient, thereby optimizing patient comfort andcompliance with the treatment. The shape or pattern for the change(ramp) in the Target Flow can be done at a fixed rate, so that the rampis linear. It can also be done at non-linear rates, so that the rampshape is not linear. In an exemplary embodiment, ramp in Target Flowtakes place at a rate of 0.5 lpm per breath.

In step 204, the Target Flow value is maintained at (1) a value thatcorresponds to 95% of the mean value of the Qave(max) data or at (2) avalue that corresponds to the mean value of Qave(max) minus a fixed flowrate, which ever is smaller. In an exemplary embodiment of the presentinvention, this fixed flow rate is 2 lpm, so that the Target Flow ismaintained at 95% of Qave(max) or at the mean value of Qave(max)−2 lpm,whichever is smaller. If, however, a sleep disordered breathing event,such as an apnea, hypopnea, or periodic breathing, is detected theprocess moves to step 206, where the Target Flow is changed to the60^(th) percentile. This increase in the Target Flow provides a greaterlikelihood that the system will increase the pressure support, and, thustreat the sleep disordered breathing event, than if the Target Flow isnot changed. The Target Flow is maintained at this level for a period oftime, such as one minute. After that, the process moves to step 208.

In step 208, the Target Flow is changed back to the lesser of 1) 95% ofthe mean value of the Qave(max) data currently collected or 2) the meanvalue of Qave(max) minus a fixed flow rate, such as 2 lpm. In apresently preferred embodiment, this change takes place in a linear,ramp fashion, over a period of time that spans several respiratorycycles, such as 2 minutes at a rate of 0.5 lpm per breath. The change inTarget Flow can also be done at a non-linear rate.

The system maintains the Target Flow at its current value in a holdstate in step 210. This is done to allow the patient to stabilize underthe new value for the Target Flow. This prevents the system of thepresent invention from overcompensating or being too aggressive in itsreactions to the monitored condition of the patient. In a presentlypreferred embodiment, this hold state lasts for 1.5 minutes. Of course,other periods of time can be used, and this period of time can beselected dynamically by the system. After the 1.5 minute hold, theprocess returns to step 202.

If a CSR event is detected during step 204, the process moves to step212, where the Target Flow is changed to the 60^(th) percentile. TheTarget Flow is maintained at this level for a relatively short period oftime, such as 30 seconds. (Timer 1 in FIG. 8). If no CSR events aredetected during this 30 second window, the process moves to step 208.If, however, CSR events continue to be detected, the system will waitanother 30 seconds after which the process proceeds to step 206regardless of whether further CSR events are detected. (Timer 2 in FIG.8).

It can be appreciated that the present invention is not to be limited tothe specific time periods, percentages, and constants noted above.Rather, other values for these quantities can be used so long as thegeneral principles of the present invention are maintained. In addition,these quantities need not be fixed. Instead, they can be dynamicallyaltered by the controller based on the monitored condition of thepatient. This can be done, for example, to treat the patient moreaggressively if they are not responding to the current treatment scheme,and vise versa.

FIGS. 9A-9D illustrate an exemplary operation of the pressure supportsystem of the present invention in accordance with the descriptionpresented above. More specifically, FIG. 9A illustrates patient flow230, FIG. 9B illustrates the pressure support 232 delivered to thepatient, FIG. 9C is a waveform 234 of the Target Flow, and FIG. 9D is astate chart 236 indicating the absence of a CSR event (low level) andthe detection of a CSR event (high level). It should be noted that thewaveforms shown in these figures illustrates a patient being treatedwith a pressure support level, where the Target Flow is being rampeddown (step 202) in FIG. 8.

Initially, as indicated by arrow 240, the patient experiences a CSRevent and the detection of this event is indicated by a change from alow to a high state in FIG. 9D, as indicated by state change 242. Thedetection of the CSR event causes the Target Flow to be increase, asindicated by arrow 244 in FIG. 9C, which is the operation required bystep 212 in FIG. 8. The Target Flow is maintained at this new levelduring a 30 or 60 second period 246 (step 212 in FIG. 8). Thereafter,the Target Flow is reduced beginning at point 248 (step 208 in FIG. 8).It can be appreciated that during this time period up to point 248 thepressure support delivered to the patient has been relatively aggressiveto treat the detected CSR.

After point 248, the Target flow is again ramped down to a lower level,and the patient's flow has stabilized, meaning that the CSR events havebeen reduced or eliminated. After a certain period of time, which isgenerally indicated by arrow 250, the system deems there to be no moreCSR events, and the Target Flow is set to a lower value, as indicated byarrow 252. The CSR state also changes from high to low at point 254 inresponse to the determination that the patient is no longer sufferingfrom CSR events. After point 252, the Target Flow continues to decreaseand the pressure support also decreases, as indicated by arrow 256.

D. Pressure Support/IPAP Control

The pressure support system of the present invention employs threeprimary pressure controls. In other words, the present inventioncontemplates providing three pressure control settings that are capableof being set as inputs to the pressure support system. These pressurecontrol settings can be set by anyone authorized to access suchsettings. Such people can include the user, manufacturer, medical deviceprovider, caregiver, etc.

First, the system has the ability to set the EPAP pressure to bedelivered to the patient and/or to control the expiratory pressureP_(exh) as noted above. Second, the minimum IPAP level (IPAP_(min)) canbe set. This is a pressure level below which the IPAP pressure will notdrop. Third, the maximum IPAP level (IPAP_(max)) can be set. This is apressure level that the IPAP pressure will not exceed. As will beunderstood from the description of the present invention, the IPAPpressure will vary as the pressure support system treats the patient'sCSR. The IPAP_(min) and IPAP_(max) establish the operating range for theIPAP pressure delivered to the patient. It can be appreciated that allthree of these pressure controls can be set to the same value, whichwould result in the pressure support system providing a CPAP therapywith CSR diagnostic capabilities. That is, the system would be able tomonitor the patient for CSR, but would not treat the CSR becauseIPAP_(min)=IPAP_(max)=EPAP.

Referring again to FIG. 4, in step 250, the IPAP pressure to bedelivered to the patient is determined based on 1) the currentQave(max), 2) the pressure support delivered during the previous breath,3) the Target Flow value determined in step 150, and 4) a gain factor.As noted above, the pressure support is the difference between the IPAPlevel and the EPAP level. The following algorithm is used to determinethe pressure support delivered to a patient during a current breath(k+1):PS(k+1)=PS(k)+Gain*(Target Flow−Qave(max)(k)),  (7)where k is the index of the last breath, PS(k) is the pressure supportdelivered during the previous breath, Gain is a factor that convertsflow into pressure, Target Flow is determined as discussed above, andQave(max)(k) is the Maximum Average Inspiratory Flow Qave(max) from theprevious breath.

The Gain used for spontaneous breaths is a 30 breath average of a ratioof pressure support (PS) over the Maximum Average Inspiratory Flow. Morespecifically, determining the Gain involves determining the ratio ofPS/Qave(max) for each breath over a thirty breath interval. The mean,i.e., average, value of these accumulated ratios is determined and usedas the Gain in equation (7). It can be appreciated that this Gain willbe updated every breath as a new ratio for the last breath is consideredin the 30 breath interval and the oldest ratio falls out of this window.Please note that the present invention contemplates that the window overwhich the ratios of PS/Qave(max) are accumulated can be a number otherthan 30 breaths. However, it is preferable that the number of breaths inthis window be great enough to provide reliable data, yet low enough toallow the system to respond in a timely manner to global changes in thepatient's respiratory pattern, for example, if the patient rolls overduring sleep or enters a different sleep stage.

The ratio of PS/Qave(max) over the 30 breath window is determinedseparately for spontaneously triggered breaths, i.e., breaths triggeredby the patient, and machine triggered breaths. As discussed in greatdetail below, machine triggered breaths are breaths delivered to thepatient with little or no patient effort. Machine triggered breaths areprovided based on an automatic backup breath delivery system in theevent a spontaneous breath is not taken by the patient within apredetermined period of time. It can thus be appreciated that one 30breath window includes the ratios associated only with spontaneouslyinduced breaths, and a separate 30 breath window is maintained formachine triggered breaths. This is done because the spontaneous breathdata contains the contributions provided by the patient's muscle effort,while the machine triggered breath data does not.

The process shown in FIG. 4 shows the calculations that are preformed bythe pressure support system during each breath. FIG. 10 illustrates thepressure/flow control process that is carried out during each breathusing the results of the calculations determined according to theprocess of FIG. 4. In step 300 in FIG. 10, the controller firstdetermines whether it is in the inspiratory phase of the respiratorycycle. As noted above, this is accomplished using any conventionaltechnique for differentiating between inspiration and expiration. In anexemplary embodiment of the present invention, a flag is set wheneverthe patient is in inspiration.

If the patient is in the inspiratory phase of the respiratory cycle, theprocess proceeds along path 302, and the controller causes the gasflow/pressure generator to begin to deliver the inspiratory pressureP_(insp) to the patient based on the IPAP pressure calculated in step250 of FIG. 4. The process then proceeds to steps 304 and 306, which areprocesses that control the pressure delivered to the patient during orwithin the respiratory cycle. The process implemented in step 304 isdiscussed in Section E below with reference to FIGS. 11-13, and theprocess implemented in step 306 is discussed in Section F below withreference to FIGS. 14 and 15. After the intra-breath pressure increaseand pressure control techniques in step 304 and 306 are performed forthat processing cycle, the process repeats back to step 300 along path308.

If the patient is not currently in the inspiratory phase of therespiratory cycle in step 300, the process proceeds to step 310. In thisstep, the controller determines whether the patient has now initiatedthe inspiratory cycle, i.e., there was a spontaneous inspiration, or ifthe pressure support system has taken over and delivered a machinetriggered breath. The determination of whether the patient has initiateda spontaneous breath can be accomplished using any conventionaltechnique. Preferably, a flag or other indicator is provided to allowthe system to differentiate between these two different alternatives.

If the patient is deemed in step 310 to have spontaneously triggered thesystem from the expiratory to the inspiratory phase, the pressuresupport system begins to deliver the inspiratory pressure as the IPAPpressure or according to an inspiratory pressure profile P_(insp), asnoted above, i.e., based on the IPAP pressure calculated in step 250 ofFIG. 4. The process again proceeds along path 302 to steps 304 and 306.

If it is determined in step 310 that the no spontaneous breath has beeninitiated, the process proceeds to step 312. In this step, the systemdetermines a threshold time period T_(apnea) that is used to determinewhether a machine triggered breath will be delivered. Threshold timeperiod T_(apnea) is the period of time during which the system will waitfor the patient to initiate a spontaneous inspiration. If no spontaneousinspiration is detected beforehand, at the end of the T_(apnea) period,the system will deliver a machine triggered breath to the patient. Theprocess for setting threshold time period T_(apnea) is discussed belowwith reference to Section G and with reference to FIGS. 16 and 17A-17C.

In step 314 the system compares the threshold time period T_(apnea) witha timer that was started at the last trigger, i.e., at the lasttransition (whether spontaneous or machine triggered) from expiration toinspiration. If the threshold time period T_(apnea) has not yet elapsed,the system returns to step 300 via path 308. If, on the other hand, thethreshold time period T_(apnea) has elapsed since the last trigger, thesystem delivers a machine triggered breath in step 316, and the processcontinues on to steps 304 and 306. The pressure support delivered in themachine triggered breath is determined as discussed herein with respectto FIGS. 4-14.

E. Intra-Breath IPAP Pressure Increase

The process that the present invention uses to ensure that the patientreceives adequate ventilation (pressure support) will be discussed. Thisprocess is implemented in step 304 of FIG. 10, and is shown in detail inFIGS. 11-13. For all breaths, which include both spontaneous and machinetriggered breaths, an intra-breath pressure control process shown inFIG. 11 is carried out by the pressure support system. The goal of thisprocess is to ensure that the Target Flow value calculated in step 150of FIG. 4 is obtained by the patient during each breath. It should benoted that the pressure increase from EPAP to IPAP occurs over time, notinstantaneously. In addition, in an exemplary embodiment of the presentinvention, the rate of change for this pressure increase, which istypically referred to as the rise time, is set by the user. The presentinvention also contemplates that the rise time and the shape or profileof the pressure or flow waveform during this EPAP to IPAP transition canbe controlled by the system, preferably to maximize patient comfort.

In step 400, the process determines whether the pressure supportincrease delivered thus far is sufficient. For present purposes, thepressure support delivered thus far is considered to be sufficient ifthe pressure support increase delivered by the system during theinspiratory phase under the current magnitude and rate of increase willresult in Qave(t) meeting or exceeding the Target Flow. This isdiscussed in greater detail in Section F. If it is determined that thepressure support for the breath will be sufficient, this processrepeats, as indicated by path 402. If it is determined that the pressuresupport for the breath will not be sufficient, i.e., the patient willnot receive sufficient pressure support to cause Qave(t) to meet orexceed the Target Flow, the process proceeds to step 404.

In step 404, the system determines how long the patient has been in theinspiratory phase. Determining how long the patient has been in theinspiratory phase includes determining an Average Inspiratory Time(T_(insp)(ave)) from the inspiratory phases of previous respiratorycycles. In an exemplary embodiment of the present invention,T_(insp)(ave) is determined over a five (5) minute window, so that theinspiratory periods over the last 5 minutes worth of inspiratory cyclesare averaged to calculate T_(insp)(ave). The system also calculates avalue that corresponds to half the average inspiratory times(T_(insp)(ave)/2).

As indicated by path 406 and block 408 of FIG. 11, and as shown in FIG.13, while the patient is in a period of time 412 that starts at a point415, which is 100 milliseconds (ms) prior to T_(insp)(ave)/2, asindicated at 417, the system collects flow data, i.e., the Qave(t) data.The end of time period 412 can be expressed mathematically asT_(insp)(ave)/2. The data collected during period 412(T_(insp)(ave)/2-100 ms to time T_(insp)(ave)/2), is used, as discussedbelow, to determine whether an increase in pressure/flow is needed inorder to ensure that the Target Flow is delivered to the patient.

During data collection step 408 in time period 412, the InstantaneousAverage Inspiratory Flow Qave(t) determined during each processing cycleof the microprocessor is compared to the Target Flow calculated for thatrespiratory cycle from FIG. 4. An error signal (Error) is generatedbased on this comparison during each processing cycle, and an averageerror signal is produced during time period 412. This error signal isexpressed mathematically as: Error=Target Flow−Qave(t). Negative averageerrors are ignored, meaning that the flow delivered to the patient willlikely hit the Target Flow. A positive average error, however, suggeststhat additional IPAP pressure is needed in order for the pressuresupport system to provide the Target Flow. However, the system will waituntil a period of time corresponding to T_(insp)(ave)/2 has elapsedbefore increasing the IPAP pressure.

Once the system determines that a period of time corresponding toT_(insp)(ave)/2 has elapsed from the start of inspiration, the system isgiven the ability, if necessary, to increase the IPAP pressure. This isillustrated in FIG. 11 as flow path 416. If the average error signal ispositive, it is multiplied in step 418 by the statistical ratio of thepressure support over the Qave(max) (PS/Qave(max)), which is discussedin Section D “Pressure Support/IPAP Control” above. This calculation isnecessary to convert the average error signal, which is expressed interms of flow, to a pressure level. The pressure level determined instep 418 corresponds to a value for the additional IPAP pressure thatneeds to be added to the pressure support already being delivered to thepatient. This additional IPAP pressure is applied once during theinspiratory cycle if the controller is active, i.e., if a period of timegreater than or equal to T_(insp)(ave)/2 has elapsed from the start ofinspiration. This period of time begins at point 417 in FIG. 13 and endsat the end of the inspiratory phase. The addition of this extra IPAPpressure is shown in FIG. 12A by a notch 420, where the pressureincreases from a first pressure waveform 422 to a second pressurewaveform 424. At all other times during the inspiratory phase of therespiratory cycle, the processing routine follows path 410 and returnsto step 400.

F. Intra-Breath IPAP Pressure Control

Referring now to FIGS. 14 and 15A-15C, an intra-breath pressure controltechnique implemented in step 306 of FIG. 10 will now be discussed.FIGS. 15A and 15B show an exemplary pressure waveform 464 and flowwaveform 466, respectively, during an intra-breath pressure controlsequence.

Within each breath, the Instantaneous Average Inspiratory Flow (Qave(t))470 is collected to predict if Qave(t) will exceed Target Flow 472. Ifit is determined that under the current magnitude and rate of increase,the Qave(t) will exceed the Target Flow, then the pressure support (PS)will only increase beyond the current value if it is needed in order tosatisfy the IPAP minimum pressure control requirement.

The details of this intra-breath pressure control technique are asfollows. After the start of a new breath, i.e., at the trigger fromexpiration to inspiration, Qave(t) is monitored and stored into anarray. See step 450. After 50 ms, the slope of Qave(t) over a 50 msmoving window is calculated in step 452, and the calculated slope isused to predict the next amplitude for Qave(t) 50 ms into the future.This is shown graphically in FIG. 15B as the slope of Qave(t) takenbetween points 474 and 476. The predicted next amplitude for Qave(t) 50ms into the future is shown as predicted amplitude 478 in FIG. 15C.

This new (predicted) amplitude 478 is compared to the current TargetFlow 472 in step 454. If the predicted Qave(t) exceeds the Target Flow,i.e., if the predicted Qave(t) crosses the Target Flow, then the risetime is changed to 600 ms in step 456. That is, the rate of pressureincrease is changed in step 456. This will slow down the pressurecontroller to allow further monitoring of Qave(t) and comparisons ofpredicted Qave(t) values against the Target Flow. In addition, a timeris started in step 456.

In step 458 the control compares the amplitude of the next predictedQave(t) with the Target Flow. In an exemplary embodiment of the presentinvention, this next comparison takes place 100 ms after the increase inthe rise rate. That is, the timer started in step 456 is monitored todetermine if 100 ms have elapsed. If the next predicted Qave(t) exceedsor crosses the Target Flow and the 100 ms interval has elapsed, thecontroller stops the pressure increase in step 460 and does not performany further analysis of the Instantaneous Average Inspiratory Flow forthe rest of the breath. However, the pressure is allowed to increaseuntil the minimum IPAP level is reached, but additional pressure changeswill not be applied on this breath after half the inspiratory time isreached.

It is to be understood that various parameters used in determiningwhether to perform an intra-breath pressure control and the parametersassociated with the pressure control can be altered from those discussedabove. For example, the size of the moving window can be a value otherthan 50 ms, and the magnitude and/or profile of the pressure can becontrolled by the system or preset so that other pressure changes arepossible depending on whether the Target Flow will be exceed. Forexample, if it is determined that the new (predicted) amplitude 478 willexceed the Target Flow, instead of ceasing further pressure increases,the system can decrease the pressure, and this pressure decrease canfollow any desired shape and magnitude. Moreover, the change in pressurecan be made dependent on the degree by which the new (predicted)amplitude will exceed the Target Flow. For example, if the new(predicted) amplitude will only slightly exceed the Target Flow, thepressure can be held constant. If, however, the new (predicted)amplitude will exceed the Target Flow by a greater amount, the pressurecan be decreased.

G. Machine Triggered Breaths

As noted above, a characteristic of CSR is the presence of a hypopnea orapnea period 38 between the hyperpnea periods 36. See FIG. 1. Theseperiods are often referred to as central apneas, because the cessationof respiration during these intervals is not believed to be due to anoccluded airway. Historically, a machine triggered breath is issued whenthe patient has not initiated a spontaneous breath within a specifiedperiod of time. That time period has been measured from the start of thelast spontaneous breath. The timer is reset each time a new spontaneousbreath is initiated. The period or rate of breathing is sometimescontrolled by a setting on the device that specifies the rate ofbreathing the device should control and the duration of IPAP pressure tobe delivered to the patient (T_(insp)).

One problem with this conventional approach to delivering machinetriggered breaths is how to deal with a sigh breath. The timer is resetat the beginning of the sigh breath and the machine triggered breathoccurs based on the typical breathing rate. Sigh breaths are larger thanthe typical breathing period and should allow the patient to exhalelonger. Further, the increased ventilation associated with a sigh breathfurther delays the need for a breath and this also should be considered.

The present invention addresses these periods of apnea in a machinetriggered breath delivery process, which was discussed above withrespect to steps 310-316 in FIG. 10. As noted above, the machinetriggered breath process monitors the amount of time that has elapsedsince the last transition from the expiratory to the inspiratory phaseof the respiratory cycle. If no spontaneous inspiratory effort isdetected over a certain period of time, a “machine triggered breath” isautomatically delivered to the patient by the pressure support system,thus ventilating the lungs. In the presently preferred exemplaryembodiment, the apnea detection time T_(apnea) begins at the start ofeach inspiration.

The present invention resets the timer at the transition from IPAP toEPAP and allows the patient a period of time, such as 8 seconds, toinitiate a spontaneous breath before a machine triggered breath isgenerated. This could be expressed as allowing the patient 8 seconds toexhale before a machine triggered breath is generated. In this manor,the patient's sigh breaths influence the delivery of the machinetriggered breath. A larger inspiration leads to a delayed back upbreath. This invention also ignores spontaneous breaths which are lessthan 100 ml. Typically, several breaths within a decrescendo associatedwith CSR are insignificant in terms of providing ventilation to thepatient. Ignoring small breaths allows the machine triggered breath tobe delivered to the patient within an adequate period of time. Thisinvention monitors the patients spontaneous breaths to determine theoptimal breath period and time of inspiration.

One could further postulate that the end of exhalation could be detectedand the timer could be reset at the time that most expiration flow hasceased. Although useful in some cases this method does risk preciseoperation when the patient exhales out his mouth instead of the mask. Inthis case, the algorithm would not see expiration at all or would see ashort expiration. Both of which would lead to an early back up breath.Triggering a machine triggered breath too early is cumbersome anddisruptive to a patient.

FIG. 16 shows the process for selecting the apnea detection timeT_(apnea) according to the principles of the present invention, andFIGS. 17A-17C illustrate various machine triggered breath deliveryscenarios. Please note that the technique for determining whether todeliver a machine triggered breath was discussed above with respect tosteps 310-316 in FIG. 10. FIG. 16 illustrates the process for settingthe apnea detection time T_(apnea), which is done in step 312 of FIG.10. For present purposes, a tidal volume of less than 100 ml is notcounted as a breath, i.e., it is not considered a spontaneousinspiration.

The first criteria considered in setting T_(apnea) is to determinewhether the patient has already received a machine triggered breath, andif so, how many. To this end, step 500 in FIG. 16 determines whether thepatient is receiving a first machine triggered breath, meaning that thepreceding breath was a spontaneous breath, or whether the patient hasalready received 4 or less machine triggered breaths. If the patient didnot receive a machine triggered breath on the previous breath, theprocess moves to step 502. In this step, the system determines whetherthere are any recent sleep disordered breathing events. Recall thatthese events are captured in step 110 of the process shown in FIG. 4.

In an exemplary embodiment of the present invention, the systemconsiders a sleep disordered breathing event that has taken place in thelast 5 minutes to be recent. Thus, the system stores and monitors thelast 5 minutes worth of sleep disordered breathing events in deciding instep 502 whether there have been any “recent” sleep disordered breathingevents. Of course, the present invention contemplates that other periodsof time can be used to define what constitutes a recent sleep disorderedbreathing event. This time period can also be adjusted automatically bythe system.

If there have been no recent sleep disordered breathing events,T_(apnea) is set to T_(insp) plus eight (8) seconds in step 504. That isT_(apnea)=T_(insp)+8 seconds. If there have been recent sleep disorderedbreathing events, T_(apnea) is set to T_(breath) plus four (4) secondsin step 506. That is T_(apnea)=T_(breath)+4 seconds. The presentinvention contemplates that constants other than eight and four can beused in steps 504 and 506 respectively, so long as a shorter constant isused in step 506 than in 504.

Spontaneous breaths are used to compute an average breath period and anaverage inspiratory period. By setting T_(apnea)=T_(insp)+8 seconds instep 504 if there have been no recent sleep disordered breathing events,the patient is given 8 seconds to exhale before the first machinetriggered breath is delivered. In addition, because T_(apnea) is setbased on the average inspiratory period of the current spontaneousbreaths, a large inspiratory sigh breath will allow the patient moretime to exhale (pause) before issuing a machine triggered breath. Whensleep disordered breathing events are present, the first machinetriggered breath is based on T_(breath) plus 4 seconds.

If in step 500 the patient has already received 1 machine triggeredbreath, but has not received more than 5 machine triggered breaths, theprocess moves to step 508. In this situation, the time between the firstmachine triggered breath and subsequent machine triggered breaths(T_(apnea)) is set to T_(breath) plus 4 seconds (T_(apnea)=T_(breath)+4sec). This allows the patient the opportunity to resume spontaneousbreathing.

FIGS. 17A-17C illustrate various alternative situations for deliveringmachine triggered backup breaths according to the process of FIG. 16,where spontaneous breaths are indicated as light-shaded boxes, andmachine triggered breaths are indicated as dark boxes. FIG. 17A shows asituation where three spontaneous breaths 550 are delivered to thepatient, who had not suffered any recent sleep disordered breathingevents. During period 552, the patient then does not take a spontaneousbreath, i.e., suffers a long apnea, so that a machine triggered breath554 is delivered at the end of T_(apnea), which is determined asT_(insp)+8 sec. The time period for delivering the next 4 machinetriggered breath is then set to T_(breath)+4 sec. (Step 508 in FIG. 16).In this example, the patient fails to take a spontaneous breath withinthe T_(apnea) period for the next 4 breaths. After a fifth machinetriggered breath 556, the system determines whether there were recentsleep disordered breathing events. In this case there were, so T_(apnea)is set to T_(breath)+4 sec even after the fifth machine triggered breath556 (Step 506 in FIG. 16). In the illustrated example, machine triggeredbackup breaths 557 continue to be delivered at T_(breath)+4 sec.

FIG. 17B begins the same as FIG. 17A, except that after a fourth machinetriggered breath 558, the patient takes a spontaneous breath before theend of T_(apnea) (T_(breath)=4 sec). After this spontaneous breath,T_(apnea) remains T_(breath)+4 sec according to step 508 in FIG. 16. Thenext three breaths 560 are spontaneous breaths initiated by the patientbefore the expiration of T_(apnea) (T_(breath)+4 sec). However, thepatient fails to take a spontaneous breath after the last of these threebreaths before the expiration of T_(apnea) and machine triggered breath562 is delivered at T_(breath)+4 sec.

FIG. 17C illustrates a situation that is similar to that of FIG. 17B,except that the patient takes a series of spontaneous breaths 564, andthere are no recent sleep disordered breathing events, so that T_(apnea)is set to T_(insp)+8 sec (Step 504 in FIG. 16).

The present invention contemplates that controller 64 implements any ofthe standard functions of a pressure support device, i.e., providingCPAP, bi-level pressure support BiPAP, PPAP pressure support, smart-CPAPas taught, for example, in U.S. Pat. Nos. 5,203,343; 5,458,137; and6,087,747, the contents of which are incorporated herein by reference,or auto-titration CPAP as taught, for example, in U.S. Pat. No.5,645,053, the contents of which are also incorporated herein byreference, in addition to implementing the CSR treatment mode ofpressure support of the present invention. In one embodiment of thepresent invention, the pressure support system includes a mode selectinput device that allows a user or authorized caregiver to select themode of ventilation (CSR treatment technique of the present invention,CPAP, bi-level, auto-titration CPAP, PAV, PPAP, etc.) under which thepressure support device operates. In addition, the present inventioncontemplates performing the CSR detection techniques in the backgroundwhile implementing a conventional mode of pressure support and thenswitching to the CSR treatment mode of pressure support once CSR isdetected.

The present invention contemplates monitoring the leakage of gas fromthe system and using different criteria for the various parameters ofthe present invention, such as differently sized windows for computingmoving averages, depending on the size or the stability of the leakageof gas from the system. In an exemplary embodiment, determining whetherthe leak is stable involves comparing the average total patient flow toan empirically developed pressure versus flow curve to determine if theleak from the system exceeds a worse case leak. An example of thisprocess is described in the '079 application in the section of the '079application discussing the flow limit control layer and the big leakdetection layer. However, a brief description of this technique isprovided below for the sake of completeness.

A worst case leak flow curve for each operating pressure level of thepressure support system is determined in advance. Each worst case leakflow curve represents a leakage flow that corresponds to worst casesystem leak. The estimated leak for the pressure support system isdetermined using any conventional leak estimation technique. If thecurrent estimated leak is above the worst case leak flow curveassociated with the pressure at which the system is operating, theestimated leak exceeds the leakage flow that constitutes a reliableoperating range for the pressure support system. This can occur, forexample, if the patient interface device becomes partially dislodgedfrom the patient so that more gas is leaking from the patient circuitthan would otherwise be expected for the type of patient circuit beingused. If, however, the estimated leak lies on or below the worst caseleak curve, there is considered to be an acceptable level of systemleak.

H. Alternative CSR Detection Technique

Section A.1., above, describes one exemplary technique for detecting CSRsuitable for use in the present invention. This technique detects CSR bymonitoring a characteristic associated with the flow of gas to or fromthe user, such as the flow rate. Another technique for detecting CSRthat can be used alone or in combination with the technique discussedabove involves detecting CSR by monitoring the oxygen saturation of theuser. Oxygen saturation of the patient, which is referred to as SpO₂, istypically monitored using a pulse oximeter.

As shown in the state diagram of FIG. 18, the patient's SpO₂ normallyhas a baseline or average level 600. During a CSR event, the SpO₂ascends or increases, as indicated by state 602. This increase in SpO₂coincides with waxing period 32 of the CSR pattern, as the patienthyperventilates and consequently introduces more oxygen into thebloodstream. Of course, the changes in SpO₂ lag the changes inrespiration, because of the time delay associate with exchanging oxygenin the lungs. During a CSR event, the SpO₂ will decrease, as indicatedby state 604. This coincides with waning periods 34 at the end of theCSR cycle as the patient's respiratory drive decreases. The presentinvention contemplates monitoring SpO₂ to identify the ascending anddescending states that indicate that the patient has experienced a CSRcycle.

More specifically, a controller or processor receives the signals fromthe SpO₂ monitor and determines the state (600, 602, 602) the patient isin, so that a CSR pattern can be detected. In baseline state 600, thecontroller looks for an ascent or increase in SpO₂ as shown bytransition 610. The present invention contemplate that the controllerupdates the baseline SpO₂ value for later comparison to minimum SpO₂value during the cycle. If the SpO₂ remains in steady state for apredefined time after the controller has switched to the ascendingstate, then the controller returns to baseline state (transition 612)and the CSR cycle is not completed. On the other hand, if the SpO₂samples start to decrease, then the state machine is switched todescending state (transition 614). Finally, in the descending state, thecontroller switches to baseline state 600 when the steady statecondition is satisfied or when a SpO₂ threshold is reached (transition616). This threshold corresponds a minimum saturation percentage. Ineither case, the state machine returns to the initial state of baseline,starting a new cycle. If the controller completes the cycle, i.e.,completes transitions 610, 614, and 616, a CSR event is declared.

The present invention contemplates using any technique for monitoringSpO₂ to detect changes thereto. For example, the monitored SpO₂ can becompared to a threshold tolerance level before the controller deems theSpO₂ to be changing. That is, no change will be declared unless thecurrent SpO₂ is more or less than a tolerance threshold. In addition,the controller can require that a certain number of changes in a row bedetected before an increase or decrease is declared. In addition,changes in SpO₂ can be detected based on absolute or relative values. Itcan be appreciated that a vast variety of techniques can be used todetect changes in the SpO₂. Thus, the present invention will not attemptto list or describe all of the myriad of change detecting techniques forthe sake of brevity.

If not otherwise stated herein, it may be assumed that all componentsand/or processes described heretofore may, if appropriate, be consideredto be interchangeable with similar components and/or processes disclosedelsewhere in the specification, unless an indication is made to thecontrary.

Although the invention has been described in detail for the purpose ofillustration based on what is currently considered to be the mostpractical and preferred embodiments, it is to be understood that suchdetail is solely for that purpose and that the invention is not limitedto the disclosed embodiments, but, on the contrary, is intended to covermodifications and equivalent arrangements that are within the spirit andscope of the appended claims. For example, it is to be understood thatthe present invention contemplates that, to the extent possible, one ormore features of any embodiment can be combined with one or morefeatures of any other embodiment.

What is claimed is:
 1. A system for delivering a flow of gas to anairway of a patient, the apparatus comprising: a gas flow/pressuregenerating system that generates a flow of gas; a patient circuitcoupled to the gas flow/pressure generating system and adapted tocommunicate the flow of gas to an airway of a patient; a sensor adaptedto monitor a characteristic that varies based on variations of the flowof the breathing gas; and a controller configured to (a) determine aTarget Flow to be delivered to a patient based on the monitoredcharacteristic, (b) determine whether such a patient is experiencing asleep disordered breathing event, (c) alter the Target Flow based on adetermination that such a patient is experiencing a sleep disorderedbreathing event, and (d) control the gas flow/pressure generating systemso as to (1) during a current breath make a determination as to whetherthe Target Flow will be exceeded during the current breath, (2) increasea pressure of the flow of breathing gas provided by the gasflow/pressure generating system during the current breath responsive toa determination that the Target Flow will not be exceeded in the currentbreath, and (3) prevent an increase in the pressure of the flow ofbreathing gas during the current breath responsive to a determinationthat the Target Flow will be exceeded during the current breath.
 2. Thesystem of claim 1, wherein the gas flow/pressure generating systemincludes: a pressure generator adapted to generate a flow of gas; and apressure control valve associated with the pressure generator or thepatient circuit to control a pressure of the flow of gas delivered to apatient by the patient circuit.
 3. The system of claim 1, wherein thecontroller further: monitors a first amount of time that has elapsedbetween (a) a transition from an expiratory phase to an inspiratoryphase of a respiratory cycle and (b) a transition from the inspiratoryphase to an expiratory phase of the respiratory cycle (T_(insp));determines an apnea detection time T_(apnea) as T_(insp)+a constant;monitors a second amount of time that has elapsed since the transitionfrom the expiratory phase to the inspiratory phase of a respiratorycycle; compares the second amount of time to T_(apnea); and delivers amachine triggered breath responsive to the second amount of timereaching T_(apnea).
 4. The system of claim 3, where the controlleradjusts the constant based on whether such a patient has experienced asleep disordered breathing event.
 5. The system for claim 1, wherein thecontroller decreases the pressure of the flow of breathing gas providedby the gas flow/pressure generating system below a prior expiratorypositive airway pressure (EPAP) responsive to a determination that theTarget Flow will be exceeded.
 6. The system of claim 1, wherein theTarget Flow is set to a level sufficient to treat Cheyne StokesRespiration (CSR), and wherein the controller determines acharacteristic associated with patient respiration, and detects CSR bycomparing the characteristic to a CSR template.
 7. The system of claim1, wherein the controller determines a Maximum Average Inspiratory Flow(Qave(max)) determined based on the monitored characteristic, whereinthe controller compares the Qave(max) to the Target Flow, and whereinthe controller controls the gas flow/pressure generating system based ona result of this comparison.
 8. The system of claim 7, wherein theMaximum Average Inspiratory Flow (Qave(max) is used to control the flowof gas during at least a portion of an inspiratory phase of arespiratory cycle.
 9. A method of delivering a flow of pressurized gasto an airway of a patient, the method comprising the steps of:delivering a flow of gas to the airway of the patient from a source ofgas via a patient circuit; monitoring a characteristic that varies basedon variations of the flow of the gas; determining a Target Flow to bedelivered to the patient based on the monitored characteristic;determining whether such a patient is experiencing a sleep disorderedbreathing event; altering the Target Flow based on a determination thatsuch a patient is experiencing a sleep disordered breathing event;determining during a current breath whether the Target Flow will beexceeded during the current breath; increasing a pressure of the flow ofbreathing gas provided to the patient during the current breathresponsive to a determination that the Target Flow will not be exceededduring the current breath; and preventing an increase in the pressure ofthe flow of breathing gas to the patient during the current breathresponsive to a determination that the Target Flow will be exceededduring the current breath.
 10. The method of claim 9, wherein deliveringa flow of gas includes generating a flow of breathing gas via a pressuregenerator and controlling a pressure of the flow of gas via (1) apressure control valve associated with the pressure generator or thepatient circuit, (2) controlling an operating speed of the pressuregenerator, or (3) a combination of both (1) and (2).
 11. The method ofclaim 9, further comprising: monitoring a first amount of time that haselapsed between (a) a transition from an expiratory phase to aninspiratory phase of a respiratory cycle and (b) a transition from theinspiratory phase to an expiratory phase of the respiratory cycle(T_(insp)); determining an apnea detection time T_(apnea) as T_(insp)+aconstant; monitoring a second amount of time that has elapsed since thetransition from the expiratory phase to the inspiratory phase of arespiratory cycle; comparing the second amount of time to T_(apnea); anddelivering a machine triggered breath responsive to the second amount oftime reaching T_(apnea).
 12. The method of claim 11, further comprising:determining whether such a patient has experienced a sleep disorderedbreathing event; and adjusting the constant based on whether such apatient has experienced a sleep disordered breathing event.
 13. Themethod of claim 9, wherein preventing an increase in the pressure of theflow of breathing gas responsive to a determination that the Target Flowwill be exceeded includes decreasing the pressure of the flow ofbreathing gas provided by the gas flow/pressure generating system belowa prior expiratory positive airway pressure (EPAP) responsive to adetermination that the Target Flow will be exceeded.
 14. The method ofclaim 9, wherein determining whether such a patient is experiencing asleep disordered breathing event includes: determines a respiratorycharacteristic associated with patient respiration based on thecharacteristic; and detecting CSR by comparing the respiratorycharacteristic to a CSR template.
 15. The method of claim 9, furthercomprising: determining a Maximum Average Inspiratory Flow (Qave(max))determined based on the monitored characteristic; comparing theQave(max) to the Target Flow; and delivering the flow of gas based on aresult of this comparison.
 16. The method of claim 15, wherein theMaximum Average Inspiratory Flow (Qave(max) is used to control the flowof gas during at least a portion of an expiratory phase of a respiratorycycle.
 17. The method of claim 15, wherein the controlling comprisescontrolling the flow of breathing gas based on the Target Flow, theMaximum Average Inspiratory Flow (Qave(max) and a gain factor comprisinga ratio based on pressure support levels and Maximum Average InspiratoryFlow levels for a number of breaths.